In digital mobile communications networks and in PLMNs (Public Land Mobile Network), for example in a GSM or an UMTS network, a call to a communications unit, by means of which a specific service is requested, is generally signaled in advance in a so-called call set-up phase to the communications unit. This means that specific information is transmitted for the call and for the service. Requested services may in this case be, for example, a speech service, a data service or a fax service. Prior signaling is used to provide a network and/or a terminal, such as a mobile communications unit, with the capability to check whether the requested service can be supported or whether a user is in fact authorized to use a specific service. Furthermore, the appropriate precautions for setting up the call can be taken on the basis of call signaling such as this. The information which is specific for a call or for a service which is being requested by a call is in general signaled in a signaling message, a so-called call set-up message.
In this case, it is in turn associated with well-defined information elements, depending on the nature of the information. One of the information elements contains all of the call-specific and/or service-specific bearer capabilities, and this is referred to as the BC (Bearer Capability). A second information element is referred to as LLC (Low Layer Compatibility), and a third is referred to as HLC (High Layer Compatibility). These information elements must be provided by the communications unit from which the call originates. While LLC and HLC information elements are transparent while they are being transported through the networks to be passed through, the BC information element may vary depending on the requirements of the individual networks. For example, different BC information elements are defined for an ISDN and a PLMN. These BC information elements are mapped onto one another at interfaces between the various networks.
One problem in this case is that the supplied information elements to be mapped are generally not complete. Either the communications unit originating the call does not provide all of the information expected by the communications unit receiving the call, or a network to be passed through is not able to transport the information. The latter is the case, for example, when the networks to be passed through include an analog network, such as a PSTN (Public Switched Telephone Network) or national versions of ISDN signaling mechanisms. In the situation where the communications unit receiving the call is a mobile communications unit, it is problematic for the mobile communications network to determine which service is being requested by the call.
GSM/UMTS Standards in this case specify two different mechanisms, with one of the mechanisms being referred to as the single-numbering scheme, and the second as the multi-numbering scheme.
In the case of the single-numbering scheme, a user or a mobile communications unit receives only one number, which is valid for all the services which the user is authorized to use. In this case, it is assumed that a requested service from a landline network can be identified, for example, by means of the information which is supplied by the information elements BC, LLC, HLC. The information element BC (referred to for short in the following text just as BC) which, by way of example, is received from ISDN, is mapped onto a PLMN BC in the PLMN, that is to say, to be more precise, in a Mobile Switching Center (MSC), Visited Mobile Switching Center (VMSC), which is responsible for the mobile communications unit at the time at which the call was set up. This PLMN BC is sent to the mobile communications unit when it is complete. According to the GSM/UMTS Standards, it is not permissible to send an incomplete BC to the mobile communications unit. If the PLMN is not able to create a complete BC, then no BC is sent to the mobile communications unit. In this case, the mobile communications unit must select the service without having any information relating to the service. If the service which is selected by the mobile communications unit does not correspond to the service coming from the landline network, then the call requesting the service is rejected. If, by way of example, the mobile communications unit selects a speech service but a data service is coming from the landline network by means of a modem, then no call is set up. The single-numbering scheme can thus be used well when the landline network supplies sufficient information to make it possible to create a complete BC.
In the case of the multi-numbering scheme, a user or a mobile communications unit receives different numbers, specifically for each service which he is authorized to use, one and only one. In consequence, the communications unit from which a call to request a service originates dials the specific service just by entering an appropriate number. A corresponding complete PLMN BC which is associated with that number, is stored in an HLR (Home Location Register) in the PLMN. In this case, each number is associated with one, and only one PLMN BC. The complete PLMN BC which identifies the requested service is sent to a mobile communications unit receiving the call. In this case, it may be passed from the HLR via a VLR (Visitor Location Register) and the Visited Mobile Switching Center (VMSC) to the mobile communications unit. The multi-numbering scheme can be used when the landline network cannot supply sufficient information.
The advantage of the single-numbering scheme is the fact that numbers are saved. The disadvantage is that, in the situation where it is not possible to create a complete BC, the mobile communications unit has to select a service without any information.
The multi-numbering scheme has the advantage that the mobile communications unit receives a complete service-specific BC. However, in this case, a very large range of numbers must be made available for one and the same user, specifically one number for each service which the user may use. This disadvantage becomes more significant as the range of numbers which can be allocated is restricted, and the number of users in the present-day networks is increasing continuously.